(1) Field of the Invention
The invention generally relates to analog circuitry. More specifically, the invention relates to a resistive device for an implemented circuit using MOS materials.
(2) Prior Art
As integrated circuits become highly miniaturized, more and more circuitry is being designed on commercially important submicron metal-oxide semiconductor (herein MOS) materials. Typically, active devices such as transistors, and passive devices such as resistors are designed on a silicon die. The resistance value of such a resistor typically depends on the resistivity of the silicon. However, as increased signal processing capability is demanded from the die, the resistivity of the silicon is decreased by increasing the doping level of the silicon. Consequently, designing a passive resistor in a die becomes impracticable because of the large silicon area consumed by the resistor to compensate for the reduced resistivity of the die. Further where the resistance values reach Megaohms, it is also impractical to use a passive resistor. Such impracticabilities often require providing resistors external to the die.
However, external resistors are disadvantageous because the resistors are subject to environmental conditions. For example, humidity could cause external resistors to deteriorate to a lesser value. Further, the board on which the die and the resistors are connected ages with time resulting in leakage paths being formed which usually causes the die to malfunction. Another disadvantage is that external resistors are rather large, thereby hindering the miniaturization of the designed integrated circuit. Furthermore, additional manufacturing steps are required to add the external resistors to the board resulting in increased manufacturing costs and quality control problems.
One method of overcoming this problem is to use active resistors, i.e., resistors made of devices such as transistors which are fabricated in the die. Such resistive devices are smaller and more economical to produce than external resistors. Further high value resistance values in Megaohms can be realized in the die. Moreover, since the die containing the resistive devices is typically encapsulated with an insulating material, the resistive devices are not exposed to the erosive effects of the surrounding environment.
One such active resistor, for example, is implemented using a field-effect transistor (FET). However, FETs built on submicron MOS materials and operating in the linear region, the linear region to be discussed later, have an operating range that is too narrow for most analog input signals that could reach up to 2 volts peak to peak. This range is further narrowed in the subthreshold region, to be discussed later, which is a desirable operating region in low power analog circuits. Accordingly, what is needed is a method and apparatus to take advantage of a transistor's ability to function as a resistor while extending the transistor's operating range to cover the full operation range of analog input signal.